The present invention relates generally to ceramic bodies and, more particularly, to plasma sprayed coatings for the purpose of increasing the corrosion and erosion resistance of these ceramic bodies in molten metal and slag environments.
In the continuous casting of steel, special high refractory ceramic parts are employed to control the flow of molten metal and to protect the metal from oxidation as it is poured from ladles to tundishes and thence to the continuous casting molds. These refractory components include slide gate plates and stopper rods used in molten metal flow control, various collector nozzles in ladles and tundishes, and protective ladle shrouds and submerged pouring nozzles and subentry shrouds employed to protect the molten steel from oxidation. These specialty ceramic parts are subjected to harsh operating conditions such as severe thermal shock at casting start-up, molten steel erosion and slag corrosion attack. All of these aforementioned ceramic parts are either continuously or periodically exposed to the severe corrosive and erosive effects of the molten metal with the most severe attack occurring on the outer surfaces of the submerged shrouds and nozzles which are in direct contact with the highly corrosive molten slag layer, commonly referred to as the "slagline" area. It is, of course, extremely undesirable, from an economic standpoint, to shut down a continuous casting sequence due to the premature failure of a submerged pouring nozzle or subentry shroud. It is, therefore, highly desirable to extend the service life of these ceramic nozzles and shrouds so as to maximize casting sequence times and resulting process economics.
Heretofore the problems connected with cracking, spalling, corrosion and erosion of these ceramic parts has to a degree been somewhat decreased through the use of highly resistant materials such as stabilized zirconia (ZrO.sub.2)/graphite or magnesium oxide (MgO). These materials are, however, significantly more costly than more commonly used alumina graphite or fused silica (SiO.sub.2).
It is a known practice in the materials science/ceramics art to manufacture refractory bodies, such as a submerged pouring nozzle, for example, as a composite structure in order to increase the service life of the nozzle. The body of the nozzle may be of a carbon bonded alumina and graphite refractory material with an erosion resistant, intermediate sleeve section formed of carbon bonded zirconia and graphite refractory. The body and sleeve are co-pressed in powder form and then fired. The intermediate sleeve section is located in the region where the submerged nozzle is in contact with the slag/metal interface. The co-pressed zirconia-graphite sleeve section exhibits improved slag erosion resistance compared with nozzles which are entirely of carbon-bonded alumina-graphite refractory. Conventional carbon bonded zirconia and graphite slagline sleeves, while offering high erosion resistance, unfortunately often lose mechanical strength and fracture during long casting sequences. Such premature failure results in a shortened casting sequence which is uneconomical and is particularly burdensome since the nozzle must usually be replaced prior to its projected erosion life. It has been observed that the fractured sleeve still contains a large proportion of refractory which has not been eroded but becomes useless due to the decreased mechanical strength caused by a crystallographic change in the structure of known zirconia-graphite refractories. Thus, in the case of the submerged pouring nozzle, it would be beneficial if the useful service life of a slagline sleeve could be increased by avoiding the phase transformation cracking while, at the same time, retaining a high resistance to slag erosion during service.
It has also been observed that the corrosion and erosion resistance can be increased by decreasing the porosity of the ceramic body to improve attack against gaseous and liquid phases at high temperatures. Unfortunately, when the porosity of a ceramic body is increased to improve corrosion and erosion properties, there is generally a corresponding decrease in the thermal shock properties of the body. Finding a compromise solution to this perplexing problem has heretofore been an elusive goal for those skilled in the materials science/ceramics art.
My invention solves this problem by providing a coating of a ceramic material to the outer surface of the ceramic body, such as an alumina-graphite subentry nozzle, wherein the coating has a lower porosity than the body. In this manner, the erosion and corrosion resistance of the low porosity surface facing the molten slag and metal is increased while the remaining physical properties of the body, such as thermal shock resistance, remain unimpaired by the lower porosity coating.
In addition, the present invention provides a coating for ceramic bodies which not only is more resistant to the corrosive and erosive effects of molten slag and steel but also resists thermal stress cracking problems, particularly in conjunction with low thermal expansion ceramic bodies, such as, for example, fused silica bodies which exhibit little, if any, thermal expansion.